The present invention is concerned with reference electrodes which are employed to provide the stable reference potentials required by a variety of electroanalytical techniques, such as ion selective electrode measurements, controlled potential coulometry, polarography, and the like.
A reference electrode most frequently is used in conjunction with an ion-selective electrode to measure the activity (which is a function of concentration) of a given ion in a sample solution. Consequently, the discussion which follows primarily relates to such use. It is to be understood, however, that such discussion is not intended to in any way limit the spirit or scope of the present invention.
The two electrodes, i.e., the reference electrode and the ion-selective electrode, both of which are immersed in the sample solution, typically are connected to a means for measuring the potential difference between the electrodes, e.g., an electrometer. The reference electrode provides a constant electromotive force or potential against which the potential of the ion-selective electrode is compared. The latter potential consists of a constant component from the electrochemical half-cell of the ion-selective electrode and a variable component which is the potential across the sensing membrane and which is dependent upon the activity (concentration) of the ion being measured. The variable component, then, is readily correlated with ion activity (concentration) by known means. To give accurate results, the potential of the reference electrode should not change with the composition of the sample.
The reference electrode is designed to be minimally sensitive to changes in the external, sample ionic environment. It consists of at least three components: (1) a half-cell electrode (typically a silver-silver chloride mixture), (2) a half-cell electrolyte (typically 4 M potassium chloride solution saturated with silver ions), and (3) a reference junction. The half-cell electrode and half-cell electrolyte constitute an electrochemical half-cell having a known, stable, constant electrical potential. Direct physical, and therefore electrical, contact between the half-cell electrolyte and the sample solution is established through the reference junction which usually consists of a porous ceramic plug, metal or asbestos fiber bundle, sintered plastic, or like means of achieving a fluid mechanical leak.
As used herein, the term "half-cell electrode" means the solid-phase, electron-conducting contact with the half-cell electrolyte, at which contact the half-cell oxidation-reduction reaction occurs which establishes the stable potential between the half-cell electrolyte and the contact.
In more sophisticated reference electrode designs, separate electrolytes are used for the half-cell and reference junction and these electrolytes are separated by a barrier through which ionic conduction may occur. For example, double-junction reference electrodes utilize an inner junction of porous ceramic or other material to separate dissimilar outer junction and half-cell electrolytes. Likewise, the diffusion barrier and ion-selective barrier reference electrodes, described in applicant's copending applications 233,993 Ser. Nos. and 233,996, respectively filed Feb. 12, 1981, use low-permeability barriers having either low electrolytic conductivity or high ionic specificity respectively, to prevent flow or diffusion of heavy metal ions from the half-cell electrolyte into a similar junction electrolyte (usually pure 4 M KCl) which is free of heavy metal ions. In these cases, the inner junction, diffusion barrier, or ion-selective barrier adds to the resistance of the electrode, which makes measurements using the electrode more susceptible to electrical noise.
Such electrical noise arises from extraneous, fluctuating electric and magnetic fields, or from proximate voltage and current sources, that are coupled to the measurement circuit and tend to induce spurious changes in the potential between the ion and reference electrode termini of the electrometer. Reference electrodes typically have impedances ranging from 1-20K.OMEGA., whereas ion-selective and glass pH electrodes typically have impedances ranging from 100K.OMEGA. to over 1000M.OMEGA.. Other things being equal, the latter electrodes would be expected to contribute more noise to the measurement. However, particular care is usually taken to shield the entire cable leading from the active surface of the ion sensor to the high impedance input of the electrometer. This shielding is particularly good in combination electrodes where the internal components and lead wire of the ion sensor are completely surrounded by the electrolyte of the reference portion of the electrode and by the braided shield of the electrode cable, which also serves as the reference lead. Thus, the pH "internals" and cable are almost always well protected from stray electric and magnetic fields. On the other hand, the reference lead is almost invariably exposed to the electrical environment, whereby stray electric or magnetic fields may more readily induce current flow along it. More importantly, the sample in which the electrodes are immersed is often the source of stray currents which tend to pass via the reference electrode into the negative input of the electrometer. Typically, the negative input is connected to circuit ground and has a relatively low impedance, and therefore acts as a "sink" for stray a.c. currents. In contrast, the positive input of the electrometer has relatively high impedance, and admits negligible current. Stray fluctuating currents can be injected by, for example, the random or periodic cycling of nearby electrical machinery, thermal controllers in water baths, hand waving, the insertion of charged bodies into the sample being measured, etc. Currents passing through the reference electrode impress a voltage in proportion to its impedance, thus causing the potential of the solution to change relative to the reference (negative) terminal of the electrometer. The potential of the pH or ion electrode, which is well shielded and connected to the high impedance positive input terminal of the electrometer, will simply follow the change in potential of the solution. The result is an a.c. noise component in the measurement, which is impressed primarily across the reference impedance. Since the coupling impedance of noise sources is usually quite high, the noise impressed across the reference electrode is directly proportional to its impedance.
One approach to a specialized problem of noise is disclosed in published British patent application No. 2,027,897, wherein a wire conductor extends through the central glass conduit of an ionic test cell through which an air bubble segmented test solution flows. The conductor provides a low impedance shunt path through the solution to reduce signal distortion due to circuit impedance fluctuations arising from interruptions of the solution path between the ionic and reference sensors by air bubbles. This addresses a completely different noise source problem than that developed above, however, and would not be effective in eliminating noise due to the reference electrode impedance, and would be inapplicable to a reference electrode environment due to, inter alia, the different redox potentials of the half-cell, junction electrolyte, and test solutions.